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Public Health England (PHE) is urging people to look out for elderly relatives and friends who may be susceptible to the high temperatures - and to take extra care of young children, babies and those with heath issues.

With the warm weather expected to last until the end of Wednesday, PHE is advising a common-sense approach to dealing with the heatwave.

People are urged to drink plenty of water and avoid dehydrating alcoholic and caffeinated drinks, close curtains to keeps rooms cool, open windows and avoid people and animals being left in closed, parked vehicles which can get extremely hot.

There are also warnings to stay out of the sun between 11am and 3pm, avoid physical exertion at the hottest times of the day and stay in the shade - and to put on a hat and wear sunscreen.

Dr Angie Bone, PHE's head of extreme events, said: "For some people - such as older people, those with underlying health conditions and those with young children - summer heat can bring real health risks.

"This summer we're urging people to keep an eye on those at risk and, if you're able, offer help to stay cool and hydrated."

The sun-shone for picnickers in Primrose Hill (Image: Getty)

Temperatures could reach 35C later today (Image: Getty)

St John Ambulance has also issued advice, urging people to be aware of headaches, dizziness and cramp which can be signs of heat exhaustion and heat stroke.

Pollen levels will be high over the country for the next few days, causing havoc for hay fever sufferers - and there may be another restless night on Tuesday as temperatures reach 22C (71.6F) overnight.

The hot weather has been caused by a warm plume of air that has worked its way northwards from Spain, the Met Office said.

People enjoying sunshine and warm weather at lunchtime in Green Park (Image: LNP)

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Wednesday will see highs reaching the low 30Cs, but by Thursday fresher conditions from the Atlantic will bring temperatures down to the more comfortable mid-20Cs, reaching around 22C (71.6F) to 24C (75.2F) by Friday and Saturday.

Met Office forecaster Sophie Yeomans said: "We are looking at probably the hottest day of the year today with quite a few places reaching 34C or even 35C in the South East.

"With all the warm and humid air it does mean we are going to have a thundery breakdown later on this evening. The only part to escape will be the very south of England, with lightning, hail, gusts of wind and 30mm of rain falling in the hour, but this won't be persistent. There's potential for nearly a month's rain in some areas during the time we have the warning."